Typically, golf balls are classified as either solid or wound. Solid golf balls commonly include a core encased by a cover. Wound golf balls are generally constructed from a liquid or solid center encased by tensioned elastomeric material and a cover. In solid golf balls, the core may be either single layered or have multiple layers. Similarly, the cover may also be single or multi-layered. Sometimes, an intermediate layer is disposed between the core and the cover in a solid golf ball as well.
By tailoring characteristics such as initial velocity, spin, feel, resilience, compression, durability, flexural and/or inertial properties, a golfer's performance may be maximized. Differing weather conditions, terrain as well as individual playing styles or abilities make it desirable for manufacturers to have cores which exhibit a wide range of properties. A ball having a high spin rate makes it easier for a player to control and stop the ball. Generally, a golf ball having a hard core and a soft cover will have a high spin rate. On the other hand, a ball having a low spin rate and high resiliency will maximize distance. Here, a golf ball having a hard cover and a soft core will have a low spin rate. Meanwhile, golf balls having a hard core and a hard cover may have very high resiliency for distance, but generally have a hard feel and can be difficult to control around the greens. Accordingly, it is desirable to provide a golf ball which provides the benefits of a harder ball without sacrificing control.
Golf ball cores and/or centers may be constructed with a thermoset rubber, such as a polybutadiene-based composition. The cores can be heated and crosslinked to create certain characteristics, such as higher or lower compression, which can also impact the spin rate of the ball and/or provide better “feel.”
The prior art is comprised of various golf balls that have been designed to provide optimal playing characteristics. For example, manufactures have attempted to achieve all the desirable golf ball characteristics discussed above by providing a hardness gradient within the golf ball. U.S. Pat. No. 6,786,838 of Sullivan et al. discloses a golf ball having a core with multiple core layers such that the hardness either increases or decreases from the innermost core layer to the outermost core layer.
However, none of the prior art discloses a core comprising a single layer and having predetermined/specific regions of increasing hardness from the core center to the core outer surface. There also remains a need to achieve a single layer core that has a generally soft-to-hard gradient (a “negative” gradient), from the surface to the center with varying hardness, and to achieve a method of producing such a core that is inexpensive and efficient. A core exhibiting such characteristics would allow the golf ball designer to create products with unique combinations of compression, “feel,” and spin and would also provide substantial manufacturing costs savings by eliminating, for example, the need to laminate, mold, connect or otherwise join together or unite individual/separate core layers.